Phase Change Fluid Spring and Method for Use of Same

ABSTRACT

A phase change fluid spring ( 450 ) for actuating a downhole tool ( 50 ) in a wellbore. The phase change fluid spring ( 450 ) includes a housing ( 114 ) defining a fluid chamber ( 120 ) and a phase change fluid disposed within the fluid chamber ( 120 ). The phase change fluid is in a first phase and at a first pressure at the surface. The phase change fluid is in a second phase and at a second pressure in the wellbore, the second pressure being greater than the first pressure. The phase change fluid is operable to store and release energy downhole to actuate the downhole tool ( 50 ).

TECHNICAL FIELD OF THE INVENTION

This invention relates, in general, to a fluid spring operable to storeand release energy downhole and, in particular, to a phase change fluidspring that contains a fluid that transitions from a liquid phase at afirst pressure on the surface to a gas or supercritical phase at asecond, higher pressure downhole.

BACKGROUND OF THE INVENTION

Without limiting the scope of the present invention, its background isdescribed with reference to the operation of annulus pressure responsivedownhole tools, as an example.

In oil and gas wells, it is common to conduct well testing andstimulation operations to determine production potential and enhancethat potential. Annulus pressure responsive downhole tools have beendeveloped which operate responsive to pressure changes in the annulusbetween the testing string and the wellbore casing that can sampleformation fluids for testing or circulate fluids therethrough. Thesetools typically incorporate both a ball valve and lateral circulationports. Both the ball valve and circulation ports are operable betweenopen and closed positions. Commonly, these tools are capable ofoperating in different modes such as a drill pipe tester valve, acirculation valve and a formation tester valve, as well as providing itsoperator with the ability to displace fluids in the pipe string abovethe tool with nitrogen or another gas prior to testing or retesting. Apopular method of employing the circulating valve is to dispose itwithin a wellbore and maintain it in a well test position during flowperiods with the ball valve open and the circulation ports closed. Atthe conclusion of the flow periods, the tool is moved to a circulatingposition with the ports open and the ball valve closed. The tool may beoperated by a ball and slot type ratchet mechanism which providesopening and closing of the valve responsive to a series of annuluspressure fluctuations.

To change the pressure in the annulus between the testing string and thewellbore casing, a surface pump in communication with the annuluspressurizes the fluid located in the annulus. The pressurized fluid inthe annulus acts on a chamber located in the annulus pressure responsivedownhole tool to increase the pressure of a pressurized gas in thechamber. This fluid spring is able to store the pressure for later use,for example, to operate the annulus pressure responsive downhole tool.This process may be repeated such that the annulus pressure responsivedownhole tool may be operated through a plurality of cycles.Specifically, the annulus pressure responsive downhole tool is cycled bysequentially charging the fluid spring by increasing the annuluspressure then decreasing the annulus pressure which allows the storedpressure in the fluid spring to operate the annulus pressure responsivedownhole tool as desired.

Many of these annulus pressure responsive downhole tools require severalpressurization and depressurization cycles to fully operate theirmechanisms, such as circulating valve ratchets and the like. Forinstance, some tools may require 15 or more cycles of pressurization anddepressurization in the annulus to move the tool out of the well testposition, into the circulating position and back again.

Typically, the hydrostatic pressure alone of the fluid in the annulusnear the operating location of the tool may be 10,000 psi to 20,000 psi.To counter this pressure, the fluid spring must contain a gas pressurethat is approximately equal to or exceeds that which is expected in thedownhole environment. Accordingly, current fluid springs are pressurizedwith nitrogen at the surface to pressures that approximate the downholepressure, for example, 20,000 psi. Use of such high pressures at thesurface presents a very substantial safety risk to operating personnelduring the charging and handling of these fluid springs. Moreover, ifvery high hydrostatic pressures are encountered downhole, it may becommercially unreasonable or impractical to charge the fluid springs tothese pressures at the surface.

It would be desirable, therefore, to employ a fluid spring that does notrequire such high surface charges while still providing sufficientpressure downhole to act against hydrostatic pressure in the annulus.Additionally, it would be desirable to employ a fluid spring that iscapable of storing and releasing energy in downhole environments havingvery high hydrostatic fluid pressures.

SUMMARY OF THE INVENTION

The present invention disclosed herein comprises phase change fluidspring operable for use with a downhole tool such as an annulus pressureresponsive downhole testing tool. The phase change fluid springcontaining a phase change fluid that is chargeable at the surface tosignificantly lower pressures while providing higher pressures downholeto actuate the downhole tool. The phase change fluid spring of thepresent invention achieves this result without the need of high-pressurecharging systems on the surface. In addition, the phase change fluidspring provides energy storing capacity in very high pressure downholeconditions without requiring comparative high pressure surface charging.

Broadly stated, the present invention is directed to a fluid spring foractuating a downhole tool in a wellbore. The fluid spring includes ahousing defining a fluid chamber and a phase change fluid disposedwithin the fluid chamber. The phase change fluid is in a first phase ata first pressure at the surface and a second phase at a second pressurein the wellbore, the second pressure being greater than the firstpressure.

In one embodiment of the fluid spring, the first phase is a liquid phaseand the second phase is one of a gas phase and supercritical fluidphase. In another embodiment of the fluid spring, the first pressure isless than about 1,500 psi and the second pressure is greater than 10,000psi. In yet another embodiment of the fluid spring, the fluid is atleast one of carbon dioxide, water, ammonia, diethyl ether, methane,ethane, propane, ethylene, propylene, methanol, ethanol, Freon, acetoneand mixtures of thereof.

In another aspect, the present invention is directed to a method foractuating a downhole tool disposed within a wellbore. The methodincludes filling a fluid spring with a phase change fluid at thesurface, the phase change fluid being in a liquid phase during thefilling and being maintained at a first pressure in the fluid spring atthe surface, operably associating the fluid spring with the downholetool, lowering the downhole tool and the fluid spring into the wellboreto a desired depth such that the phase change fluid transitions from theliquid phase to one of a gas phase and a supercritical phase and to asecond pressure that is higher than the first pressure, pressurizing thephase change fluid in the fluid spring downhole such that energy tostored in the fluid spring and releasing the energy stored in the fluidspring to actuate the downhole tool.

In the method, pressurizing the phase change fluid in the fluid springdownhole may be achieved by increasing the pressure in an annulussurrounding the downhole tool. Likewise, releasing the energy stored inthe fluid spring to actuate the downhole tool may be achieved byreducing the pressure in an annulus surrounding the downhole tool. Insome implementation, it may be desirable to repeat the steps ofpressurizing the phase change fluid in the fluid spring downhole andreleasing the energy stored in the fluid spring to actuate the downholetool through a plurality of positions. This may be achieved bysequentially increasing and decreasing the pressure in the annulussurrounding the downhole tool.

In a further aspect, the present invention is directed to a tool for usein a testing string that is disposed in a wellbore. The tool includes ahousing defining a central flow conducting passage. A circulating valveis disposed within the housing operable to control fluid communicationbetween the central flow conducting passage and the exterior of thehousing. A passageway valve is disposed within the central flowconducting passage operable to control fluid flow through the centralflow conducting passage. A phase change fluid spring is operablyassociated with the circulating valve and the passageway valve. Thephase change fluid spring operates in response to changes in pressure inthe annulus, wherein the phase change fluid spring contains a fluid thatis in a first phase at a first pressure at the surface and a secondphase at a second pressure in the wellbore, the second pressure beinggreater than the first pressure.

In another aspect, the present invention is directed to a tool for usein a testing string disposed in a wellbore. The tool includes a housingdefining a central flow conducting passage. An operating element isdisposed within the central flow conducting passage and is operablebetween two positions, a first position wherein the flow conductingpassage through the tool is blocked and a second position wherein theflow conducting passage is not blocked. A fluid circulating assembly isdisposed within the housing and is operable between two positions, afirst position wherein fluid communication is allowed between theannulus and the central flow conducting passage and a second positionwherein fluid communication between the annulus and the central flowconducting passage is blocked. An operating mandrel assembly is slidablydisposed within the housing and is operably associated with theoperating element and the fluid circulating assembly. The operatingmandrel assembly is operable to move between a plurality of positionssuch that the operating element and the fluid circulating assembly areactuated to configure the tool into distinct operative modes. A phasechange fluid spring is operably associated with the operating mandrelassembly. The phase change fluid spring operates in response to changesin pressure in the annulus, wherein the phase change fluid springcontains a fluid that is in a first phase at a first pressure at thesurface and a second phase at a second pressure in the wellbore, thesecond pressure being greater than the first pressure.

In one embodiment, the tool includes a pressure conducting channelwithin the housing. The pressure conducting channel is in fluidcommunication with the phase change fluid spring and the annulus forcommunicating changes in annulus pressure to the phase change fluidspring.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the features and advantages of thepresent invention, reference is now made to the detailed description ofthe invention along with the accompanying figures in which correspondingnumerals in the different figures refer to corresponding parts in which:

FIG. 1 is a schematic illustration of a phase change fluid springcoupled within a tool string operating from an offshore platformaccording to an embodiment of the present invention;

FIGS. 2A-2J are half section views of an exemplary testing tool with aphase change fluid spring of the present invention in a well test modeaccording to an embodiment of the present invention;

FIG. 3 is a front view of a ratchet slot mandrel section that has beendepicted as a flat plate of the testing tool with a phase change fluidspring according to an embodiment of the present invention;

FIGS. 4A-4B are half section views of a phase change fluid springconnected to a phase change fluid source and hydraulic fluid source forcharging the phase change fluid spring according to an embodiment of thepresent invention; and

FIG. 5 is a phase diagram for a phase change fluid used in the phasechange fluid spring according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

While the making and using of various embodiments of the presentinvention are discussed in detail below, it should be appreciated thatthe present invention provides many applicable inventive concepts whichcan be embodied in a wide variety of specific contexts. The specificembodiments discussed herein are merely illustrative of specific ways tomake and use the invention, and do not delimit the scope of the presentinvention.

Referring initially to FIG. 1, the present invention is shownschematically incorporated in a testing string deployed in an offshoreoil or gas well. Platform 2 is shown positioned over a submerged oil orgas wellbore 4 located in the sea floor 6, wellbore 4 penetratingpotential producing formation 8. Wellbore 4 is shown to be lined withsteel casing 10, which may be cemented into place. A subsea conduit orriser 12 extends from the deck 14 of platform 2 to a subsea wellhead 16,which includes a blowout preventer 18. Platform 2 supports a derrick 20thereon, as well as a hoisting apparatus 22, and a pump 24 whichcommunicates with the wellbore 4 via control conduit 26, which extendsto annulus 46 below blowout preventer 18.

A testing string 30 is shown disposed in wellbore 4, with blowoutpreventer 18 closed thereabout. Testing string 30 includes an upperdrill pipe string 32 which extends downward from platform 2 to wellhead16. Upper drill pipe string 32 is connected to a hydraulically operatedtest tree 34, below which extends intermediate pipe string 36. Slipjoint 38 may be included in string 36 to compensate for vertical motionimparted to platform 2 by wave action. Below slip joint 38, intermediatestring 36 extends downwardly to a testing tool with phase change fluidspring 50 of the present invention. Below testing tool with phase changefluid spring 50 is a lower pipe string 40, extending to a tubing sealassembly 42, which stabs into a packer 44. Above the tubing sealassembly 42 on the lower pipe string 40 is a tester valve 41, which maybe of any suitable type known in the art. When set, packer 44 isolatesupper wellbore annulus 46 from lower wellbore 48. Packer 44 may be anysuitable packer well known in the art. Tubing seal assembly 42 permitstesting string 30 to communicate with lower wellbore 48 through aperforated tail pipe 52. In this manner, formation fluids from formation8 may enter lower wellbore 48 through the perforations 54 in casing 10,and flow into testing string 30.

After packer 44 is set in wellbore 4, a formation test for testing theproduction potential of formation 8 may be conducted by controlling theflow of fluid from formation 8 through testing string 30 usingvariations in pressure to operate testing tool with phase change fluidspring 50. The pressure variations are effected in upper annulus 46 bypump 24 and control conduit 26, utilizing associated relief valves (notshown). Prior to the actual test, however, the pressure integrity oftesting string 30 may be tested with the valve ball of the testing toolwith phase change fluid spring 50 closed in the tool's drill pipe testermode. Testing tool with phase change fluid spring 50 may be run intowellbore 4 in its drill pipe tester mode, or it may be run in itscirculation valve mode to automatically fill with fluid, and be cycledto its drill pipe mode thereafter. As the ball valve in testing toolwith phase change fluid spring 50 of the present invention is opened andclosed in its formation tester valve mode, formation pressure,temperature and recovery time may be measured during the flow testthrough the use of instruments incorporated in testing string 30 asknown in the art, such as Bourdon tube-type mechanical gauges,electronic memory gauges and sensors run on wireline from platform 2inside testing string 30 prior to the test. If the formation to betested is suspected to be weak and easily damageable by the hydrostaticpressure of fluid in testing string 30, testing tool with phase changefluid spring 50 may be cycled to its displacement mode and nitrogen orother inert gas under pressure employed to displace fluids from thestring prior to testing or retesting.

It may also be desirable to treat the formation 8 in conjunction withthe testing program while testing string 30 is in place. Treatmentprograms may include hydraulically fracturing the formation or acidizingthe formation. Such a treatment program is conducted by pumping variouschemicals and other materials down the flow bore of testing string 30 ata pressure sufficient to force the chemicals and other materials intothe formation. The chemicals, materials and pressures employed will varydepending on the formation characteristics and the desired changesthought to be effective in enhancing formation productivity. In thismanner, it is possible to conduct a testing program to determinetreatment effectiveness without removal of testing string 30. Ifdesired, treating chemicals may be spotted into testing string 30 fromthe surface by placing the testing tool with phase change fluid spring50 in its circulation valve mode, and displacing string fluids into theannulus prior to opening the ball valve in the testing tool with phasechange fluid spring 50.

At the end of the testing and treating programs, the circulation valvemode of testing tool with phase change fluid spring 50 is employed, thecirculation valve opened, and formation fluids, chemicals and otherinjected materials in testing string 30 circulated from the interior oftesting string 30 are pumped back up the testing string 30 using a cleanfluid. Packer 44 is then released (or tubing seal 42 withdrawn if packer44 is to remain in place) and testing string 30 withdrawn from wellbore4.

FIGS. 2A-2J illustrate a testing tool with phase change fluid spring 50.Testing tool with phase change fluid spring 50 is shown in section,enclosing a central flow conducting passage 56. As may be appreciated byreference to the drawings, connections of components are oftencomplimented by the use of O-rings or other conventional seals. The useof such seals is well known in the art and, therefore, will not bediscussed in detail. Commencing at the top of the testing tool withphase change fluid spring 50, upper adapter 100 has threads 102 thereinat its upper end, whereby testing tool with phase change fluid spring 50is secured to drill pipe in the testing string 30. Upper adapter 100 issecured to valve housing 104 at threaded connection 106. Valve housing104 contains a valve assembly (not shown), such as is well known in theart, and a lateral bore 108 in the wall thereof, communicating withdownwardly extending longitudinal phase change fluid charging channel110.

Valve housing 104 is secured by threaded connection 112 at its outerlower end to tubular pressure case 114, and by threaded connection 116at its inner lower end to gas chamber mandrel 118. Case 114 and mandrel118 define a pressurized gas chamber 120 and an upper oil chamber 122,the two being separated by a floating annular piston 124. Channel 110 isin communication with chamber 120.

The upper end of oil channel coupling 126 extends between case 114 andgas chamber mandrel 118, and is secured to the lower end of case 114 atthreaded connection 128. A plurality of longitudinal oil channels 130spaced around the circumference of coupling 126 (one shown), extend fromthe upper terminal end of coupling 126 to the lower terminal endthereof. Radially drilled oil fill ports 132 extend from the exterior oftesting tool with phase change fluid spring 50, intersecting withchannels 130 and closed with plugs 134. The lower end of coupling 126,includes a downwardly facing lower side 127 and is secured at threadedconnection 140 to the upper end of connector housing 123.

Connector housing 123 is connected at its lower portion by threadedconnection 125 to the fluid metering assembly 142 which is constructedprimarily of upper and lower fluid flow housings 144 and 146 and ametering nut 148. While an exemplary construction for the fluid meteringassembly 142 is described herein, it is understood that otherconstructions which perform these functions may also be used.

The upper fluid flow housing 144 is connected at its lower portion bythreaded connection 154 to the lower fluid flow housing 146 which is, inturn, connected at thread 156 to ratchet case 158, with oil fill ports160 extending through the wall of case 158 and closed by plugs 162.Ratchet case 158 presents an inwardly projecting, upwardly facingannular shoulder 164 (see FIG. 2D) on its inner surface which forms andseparates an upper expanded bore 166 from a lower reduced diameter bore168 below. The expanded bore 166 defines a ratchet chamber 170.

Referring now to FIG. 2C, the lower portion of the metering nut 148 isengaged at threads 190 to the upper fluid flow housing 144. The meteringnut 148 includes an upward facing port 192 communicating with a bore 194extending downwardly in nut 148. A fluid restrictor 196 is disposedwithin the bore 194. A radially inward facing lateral hole 198 in themetering nut 148 permits fluid communication radially inward between theannular gap 182 and the inner radial separation or clearance 199 betweenthe metering nut 148 and the bypass mandrel 206. When connected,metering nut 148 and upper fluid flow housing 144, form an externalannular groove 200 having a V-shaped cross-section. Between the upperportion of the metering nut 148 and the upper fluid flow housing 148lies fluid passage 195 which extends between the groove 200 above andupper annular gap 182 below. An elastomeric O-ring 202 is seated withinthe groove 200 so as to block fluid entry into the groove 200 andbetween the two pieces, but the O-ring 202 may be urged radially outwardby fluid pressure to permit fluid communication from the passage 195outward through the groove 200. A radial separation or clearance 204 ispresent between the metering nut 148 and connector housing 123.

The lower fluid flow housing 146 includes a pair of longitudinalpassages 172 which communicates fluid between ratchet chamber 170 belowand a lower annular gap 176 above defined at the connection of upperfluid flow housing 144 and lower fluid flow housing 146.

As depicted in FIG. 2D, on one radial side proximate its bottom portion,upper fluid flow housing 144 encases an inwardly opening non-annularcavity 178 and an adjoining annular chamber 179. The upper fluid flowhousing 144 also encases a first passage 180 which runs between an upperannular gap 182 formed between metering nut 148 and upper fluid flowhousing 144 and the non-annular cavity 178 below. A plug 184 is disposedwithin the first passage 180 just below the upper annular gap 182 so asto block fluid flow therethrough. A radially outward facing port 186within the upper fluid flow housing 144 permits fluid communicationbetween the first passage 180 and the radial clearance 204. A secondpassage 188 also communicates fluid between the lower annular gap 176and upper annular gap 182 above.

A bypass mandrel 206 (FIGS. 2B-2C) is disposed within oil channelcoupling 126, connector housing 123, and fluid metering assembly 142. Afluid chamber 129 is formed between mandrel 206 and housing 123 withcoupling 126 at its upper end and metering assembly 142 at its lowerend. One or more upper bypass grooves 208 are cut into the outer surfaceof bypass mandrel 206 such that, when the bypass mandrel is in its lowerposition fluid may be communicated along grooves 208 between fluidchamber 129 and lateral hole 198.

The fluid metering assembly 142 presents an upper end 150 and lower end152. The fluid metering assembly 142 includes an upward flow path and adownward flow path for communication therebetween. In operation, thefluid metering assembly 142 permits relatively unrestricted upwardmovement of fluid through upward flow path 188, but will meter fluiddownward over a period of time through the downward flow path.

When an upward pressure differential exists at the lower end 152 ofassembly 142, the fluid metering assembly 142 provides an upward flowpath which communicates fluid from the ratchet chamber 170 to fluidchamber 129 without presenting significant resistance. Traveling alongthe upward flow path, fluid enters passages 172 at lower end 152 and iscommunicated into the lower annular gap 176, then upward within thesecond passage 188 of upper fluid flow housing 144 to upper annular gap182. Fluid then enters passage 195 and flows radially outward throughthe V-shaped groove 200, through the clearance 204 and into fluidchamber 129. Fluid will displace the O-ring 202 much more easily than itcan pass through fluid restrictor 196, and flow past the O-ring 202presents no significant restriction.

When a downward pressure differential exists at upper end 150, the fluidmetering assembly 142 provides a downward flow path to communicate fluiddownward from fluid chamber 129 to ratchet chamber 170. The downwardflow path, unlike the upward path, provides flow resistance. By way ofexplaining the downward flow path, fluid movement within the meteringassembly 142 is described as follows. Fluid first enters the radialclearance 204 surrounding the metering nut 148. Being blocked from entryinto the groove 200 by the O-ring 202, the fluid passes further downwardthrough the clearance 204 and enters the port 186 to move into anddownward through the first passage 180 to the non-annular cavity 178 andnon-annular chamber 179. As the fluid cannot progress beyond thenon-annular gap 178 and chamber 179, it must instead take an alternatepath in which it passes downwardly through the upwardly facing port 192,bore 194 and fluid restrictor 196 to enter the upper annular gap 182where it is transmitted to the second passage 188 of upper fluid flowhousing 144 and downward to the lower annular gap 176 and can then moveinto ratchet chamber 170 through passages 172.

An annular piston 210 (FIG. 2C) is disposed within the fluid chamber 129and affixed by lock rings 212 to bypass mandrel 206 to be axiallymoveable therewith. Piston 210 includes a longitudinal bore 211therethrough having upper and lower enlarged diameter portions. An uppercheck valve 214 with an upwardly extending dart 216 within its upper endis disposed within the upper enlarged portion of bore 211. The uppercheck valve 214 is spring biased into a normally closed position whichblocks upward fluid flow across it through the piston 210 but willpermit downward fluid flow under pressure. Downward force upon the dart216 will open the upper check valve to permit upward fluid flowtherethrough. Lower check valve 218 is oppositely disposed from theupper check valve 214 within the lower enlarged portion of bore 211 ofpiston 210 and carries a downwardly extending dart 220 within its lowerend. It is spring biased into a normally closed position againstdownward fluid flow, but will permit upward fluid flow under pressure.Upward force upon the dart 220 will open the lower check valve 218 todownward fluid flow therethrough.

The bypass mandrel 206 is axially slidable with respect to the oilchannel coupling 126, housing 123, fluid chamber 129 and the fluidmetering assembly 142 between an upper position proximate the lower endof gas chamber mandrel 118 and a lower position proximate the upper endof ratchet slot mandrel 222. Ratchet slot mandrel 222 extends upwardfrom within ratchet case 158. The upper exterior 224 of ratchet slotmandrel 222 has a reduced, substantially uniform diameter, while thelower exterior 226 has a greater diameter so as to provide sufficientwall thickness for ratchet slots 228. Ratchet slot mandrel 222 includesan annular member 231 projecting radially outward and forming a pistonseat 230 which faces upwardly and outwardly at the base of the upperexterior 224 of mandrel 222. There are preferably two such ratchet slots228 extending longitudinally along the lower exterior of the ratchetslot mandrel 222.

The ratchet slot mandrel 222 is axially slidable within testing toolwith phase change fluid spring 50 between upper and lower positions aswill be described in greater detail shortly. Lower longitudinal bypassgrooves 232 are cut into the upper exterior 224 of ratchet slot mandrel222. The grooves 232 should be of sufficient width to permit fluid flowtherealong. The lower bypass grooves 232 generally adjoin the lowerfluid flow housing 144 and should be in such a location and of such alength that when the ratchet slot mandrel 222 is in its upper positions,the grooves 232 are located alongside the lower fluid flow housing 146and no fluid flow occurs along the grooves. As the ratchet slot mandrel222 is moved toward its lower positions, the grooves 232 will be moveddownward such that fluid communication may occur between the annularchamber 179 and the ratchet chamber 170.

A ball sleeve assembly 234 surrounds ratchet slot mandrel 222 andcomprises shuttle piston 236, upper sleeve 238, lower sleeve 240, andclamp 242 which connects sleeves 238 and 240.

Shuttle piston 236 is constructed similarly in structure and function toannular piston 210 and is fixedly attached to or unitarily fashionedwith upper sleeve 238. The shuttle piston 236 surrounds the upperexterior 224 of the ratchet slot mandrel 222 within the ratchet chamber170. Shuttle piston 236 includes a longitudinal bore 237 therethroughhaving upper and lower enlarged diameter portions. An upper check valve244 with upwardly extending dart 246 within its upper end is disposed inthe upper enlarged portion, and lower check valve 248 with downwardlyextending dart 250 within its lower end is disposed within the lowerenlarged portion. The lower check valve 248 and dart 250 are shown asangled outwardly within the shuttle piston 236 such that the dart 250contacts shoulder 164 when ball sleeve assembly 234 is moved downwardwithin the ratchet case 158.

The lower end 252 of the ratchet slot mandrel 222 is secured at threadedconnection 254 to extension mandrel 256. A radial clearance 258 ispresent between the radial exterior of lower end 252 and the interiorsurface of ratchet case 158. The lower end 260 of ratchet case 158 issecured at threaded connection 262 to extension case 264 which surroundsthe extension mandrel 256. Annular intermediate oil chamber 266 isdefined by ratchet case 158 and extension mandrel 256. The intermediateoil chamber 266 is connected by oil channels 268 to lower oil chamber270. Annular floating piston 272 slidingly seals the bottom of lower oilchamber 270 and divides it from the lower wall fluid chamber 274 intowhich pressure ports 282 in the wall of case 264 open.

The general construction and operation of ratchet-type assemblies iswell known in the art. As will be appreciated by the discussion thatfollows, the testing tool with phase change fluid spring 50 of thepresent invention incorporates a ratchet assembly having a dual-pathratchet slot within which a ratchet member is directed. The primary pathis cyclical and maintains the tool's components in the well test mode.The secondary path is contiguous to the first path, and redirection ofthe ratchet member into the second path permits the tool's components tobe altered so that the tool may be reconfigured into alternative modesof operation.

Referring now to FIGS. 2E and 3, two ratchet balls 276 are found in ballseats 278 located on diametrically opposite sides of lower sleeve 240and each project into a ratchet slot 228 of semi-circular cross-section.The configuration of ratchet slot 228 is shown in FIG. 3. As shownthere, the ratchet slot 228 includes an installation groove 281 whichhas a depth greater than that of the ratchet slot 228 to permit theintroduction and capture of balls 276 during assembly of the testingtool with testing tool with phase change fluid spring 50. The ratchetslot 228 includes a unique pattern or configuration having a number ofball positions, a, b, c, d₁, d₂, e₁, e₂, f₁, f₂, f₃, f₄, f₅, f₆ and f₇.The ball positions correspond to the general positions for balls 276along ratchet slot 228 during the various operations involving annuluspressurization changes. As the balls 276 follow the path of slot 228,lower sleeve 240 rotates with respect to upper sleeve 238, and axialmovement of the ball sleeve assembly 234 is transmitted to ratchet slotmandrel 222 by balls 276.

Referring again to FIG. 2G, the lower end of extension case 264 includesoil fill ports 284 containing closing plugs 286. A nipple 288 isthreaded at 290 at its upper end to extension case 264 and at 292 at itslower end to circulation displacement housing 294. The circulationdisplacement housing 294 possesses a plurality of circumferentiallyspaced, radially extending circulation ports 296, as well as one or morepressure equalization ports 298, extending through the wall thereof. Acirculation valve sleeve 300 is threaded to the lower end of extensionmandrel 256 at threaded connection 302. Valve apertures 304 extendthrough the wall of circulation valve sleeve 300 and are isolated fromcirculation ports 296 by annular seal 306, which is disposed in sealrecess 308 formed by the junction of circulation valve sleeve 300 and alower operating mandrel 310, the two being threaded together at 312.Operating mandrel 310 includes a reduced diameter, downwardly extendingskirt having an exterior annular recess 314.

A collet sleeve 318, having collet fingers 320 at its upper endextending upwardly therefrom, engages the downwardly extending skirt 316of operating mandrel 310 through the accommodation of radially, inwardlyextending protuberances 322 received by annular recess 314. As isreadily noted in FIGS. 2H-2I, protuberances 322 and the upper portionsof collet fingers 320 are confined between the exterior of mandrel 310and the interior of circulation displacement housing 294 therebymaintaining the connection.

Collet sleeve 318 includes coupling 324 at its lower end comprisingradially extending flanges 326 and 328, forming an exterior annularrecess 330 therebetween. A lower coupling 332 comprises inwardlyextending flanges 334 and 336 forming an interior recess 338therebetween and two ball operating arms 338. Couplings 324 and 332 aremaintained in engagement by their location in annular recess 340 betweenball case 342, which is threaded at 344 to circulation-displacementhousing 294, and ball housing 346. Ball housing 346 is of substantiallytubular configuration, having an upper smaller diameter portion 348 anda lower, larger diameter portion 350. Larger diameter portion 350 hastwo windows 352 cut through the wall thereof to accommodate the inwardprotrusion of lugs 354 on each of the two ball operating arms 338.Windows 352 extend from shoulder 356 downward to shoulder 358 adjacentthreaded connection 360 with ball support 362. On the exterior of theball housing 346, two longitudinal channels (location shown by phantomarrow 364) of arcuate cross-section and circumferentially aligned withwindows 352, extend from shoulder 366 downward to shoulder 356. Balloperating arms 338, which are of substantially the same arcuate crosssection as channels 364 and lower portion 350 of ball housing 346, liein channels 364 and across windows 352, and are maintained in place bythe interior wail 368 of ball case 342 and the exterior of portion 350of ball housing 346.

The interior of ball housing 346 possesses upper annular seat recess370, within which annular ball seat 372 is disposed, being biaseddownwardly against ball 374 by ring spring 376. Surface 378 of upperseat 372 comprises a metal sealing surface, which provides a slidingseal with the exterior 380 of valve ball 374.

Valve ball 374 includes a diametrical bore 382 therethrough ofsubstantially the same diameter as bore 384 of ball housing 346. Two lugrecesses 386 extend from the exterior 380 of valve ball 374 to bore 382.

The upper end 388 of ball support 362 extends into ball housing 346, andcarries lower ball recess 390 in which annular lower ball seat 392 isdisposed. Lower ball seat 392 possesses arcuate metal sealing surface394 which slidingly seals against the exterior 380 of valve ball 374.When ball housing 346 is made lap with ball support 362, upper and lowerball seats 372 and 392 are biased into sealing engagement with valveball 374 by spring 376.

Exterior annular shoulder 396 on ball support 362 is contacted by theupper ends 398 of splines 400 on the exterior of ball case 342, wherebythe assembly of ball housing 346, ball operating arms 338, valve ball374, ball seats 372 and 392 and spring 376 are maintained in positioninside of ball case 342. Splines 400 engage splines 402 on the exteriorof ball support 362, and, thus, rotation of the ball support 362 andball housing 346 within ball case 342 is prevented.

Lower adaptor 404 protrudes at its upper end 406 between ball case 342and ball support 362, sealing therebetween, when made up with ballsupport 362 at threaded connection 408. The lower end of lower adaptor404 carries on its exterior threads 410 for making up with portions of atesting tool with phase change fluid spring 50.

When valve ball 374 is in its open position, as shown in FIG. 2I, a fullopen conducting passage 56 extends throughout testing tool with phasechange fluid spring 50, providing an unimpeded path for formation fluidsand/or for perforating guns, wireline instrumentation, etc.

It is noted that an exterior housing 414 for the testing tool with phasechange fluid spring 50 may be made up of upper adapter 100, valvehousing 104, pressure case 114, oil channel coupling 126, connectorhousing 123, upper and lower fluid flow housings 144 and 146, ratchetcase 158, extension case 264, nipple 288, circulation displacementhousing 294, ball case 342 and lower adaptor 404. The ratchet slotmandrel 222, extension mandrel 256, circulation valve sleeve 300,operating mandrel 310 may be thought of as an operating mandrel assemblyindicated generally at 412.

An annulus pressure conducting channel capable of receiving, storing andreleasing annulus pressure increases is formed by pressure ports 282,fluid chamber 274, floating piston 272, lower oil chamber 270, oilchannels 268, intermediate oil chamber 266, ball sleeve assembly 234,ratchet chamber 170, fluid metering assembly 142, fluid chamber 129,longitudinal oil channels 130, upper oil chamber 122, floating piston124 and pressurized gas chamber 120. The pressurized gas chamber 120functions as a fluid spring while the other components of the pressureconducting channel serve as a pressure conducting passage to communicatefluid pressure changes between the annulus 46 and the fluid spring.

The circulation valve sleeve 300, valve apertures 304, annular seal 306,circulation displacement housing 294, and circulation ports 296 may bethought of as a fluid circulating assembly 416 which may be selectivelyopened and closed to permit fluid flow between the annulus 46 and thecentral flow conducting passage 56 of the testing tool with phase changefluid spring 50.

Referring to FIG. 1-3, operation of the testing tool with phase changefluid spring 50 of the present invention is described hereafter. Astesting tool with phase change fluid spring 50 is run into the well intesting string 30, it is normally in its well test mode as shown in FIG.2, with ball 374 in its open position and ball bore 382 aligned withtool bore 384. Circulation ports 296 are misaligned with circulationvalve apertures 304, seal 306 preventing communication therebetween.With respect to FIG. 3, balls 276 will be proximately in position a inslot 228 as testing tool with phase change fluid spring 50 is run intothe wellbore.

Pressure is increased in annulus 46 by pump 24 via control conduit 26.This increase in pressure is transmitted through pressure ports 282(FIG. 2G) into well fluid chamber 274, where it acts upon the lower sideof floating piston 272. Floating piston 272, in turn, acts upon a fluid,such as silicon oil, in lower chamber 270, which communicates via oilchannels 268 with intermediate oil chamber 266. Fluid pressure in theintermediate oil chamber 266 flows around the lower end 252 of theratchet slot mandrel 222 to exert upward fluid pressure upon the shuttlepiston 236 which pulls ball sleeve assembly 234. Balls 276 move alongslot 228 to position b via the association of the ratchet slot mandrel222 and ball sleeve assembly 234, the ratchet slot mandrel 222 and theentire operating mandrel assembly 412 may be moved upward slightly butnot a sufficient amount to affect either the valve ball 374 or thecirculating assembly 416.

Fluid within ratchet chamber 170 is evacuated upward through the fluidmetering assembly 142. By virtue of the upward flow path describedabove, the fluid is communicated into fluid chamber 129 withoutsignificant flow restriction. Annular piston 210 and the affixed bypassmandrel 206 are moved axially upward. Fluid above the piston 210 isevacuated upward from the fluid chamber 129 through longitudinalchannels 130 into upper oil chamber 122 to urge floating piston 124upward, thereby pressurizing the gas in chamber 120 to store thepressure increase.

As annulus pressure is subsequently bled off during depressurization,the pressurized gas in chamber 120 pushes downward against floatingpiston 124, this pressure is transmitted through fluid within upper oilchamber 122, channels 130 and fluid chamber 129. Annular piston 210 andthe affixed bypass mandrel 206 are moved axially downward. Fluid fromchamber 129 is transmitted downward into the ratchet chamber 170 throughthe downward flow path of the fluid metering assembly 142. Ball sleeveassembly 234 is, therefore, biased downwardly with ratchet balls 276following the paths of slot 228 past position c, where they shoulder atposition a. Downward travel of the ball sleeve assembly 234 is limitedby engagement of the shuttle piston 236 on piston seat 230 (FIG. 2D).Again, any downward movement of the ratchet slot mandrel 222 and theoperating mandrel assembly 412 will be slight and not sufficient toclose the valve ball 374 or close the circulating assembly 416. As aresult, the ratchet assembly may be thought of as providing a defaultposition sequence with the well test position cycle 283 wherein theoperating mandrel assembly 412 is maintained during annulus pressurechanges in primary mandrel positions such that the valve ball 374 andthe circulating assembly 416 are not affected.

As testing tool with phase change fluid spring 50 travels down to thelevel of the production formation 8 to be tested, at which positionpacker 44 is set, floating piston 272 moves upward under hydrostaticpressure, pushing ball sleeve assembly 234 upward and causing balls 276to move toward position b. This movement does not change tool modes oropen any valves. Upon testing tool with phase change fluid spring 50reaching formation 8, packer 44 is set. The aforesaid feature isadvantageous in that it permits pressuring of the wellbore annulus 46 totest the seal of packer 44 across the wellbore 4 without closing valveball 374. It also permits independent operation of other annuluspressure responsive tools within testing string 30.

Increases in annulus pressure will move floating piston 272 and ballsleeve assembly 234 further upward, its movement ultimately beingrestricted by the shouldering out of balls 276 at ball position b withinslot 228. Reduction in annulus pressure will move floating piston 272and ball sleeve assembly 234 downward and cause balls 276 to movedownward proximate ball position c and ultimately back to ball positiona. The well annulus pressure may be increased and decreased as manytimes as desired without moving the testing toot with phase change fluidspring 50 out of the well test position, the balls 276 following thedescribed well test position path 283, which is made up of the ballpositions a, b, and c and the paths of slot 228 connecting them.Effectively, the well test position path 283 affords default positioncontrol for the testing tool with phase change fluid spring 50 bymaintaining it in its well test position during regular annuluspressurization cycles.

The testing tool with phase change fluid spring 50 may be changed out ofthe well test position by increasing annulus pressure during the portionof the annulus pressure reduction sequence when balls 276 are proximateball position c. As a result, annulus repressurization during a releaseof stored fluid pressure from the pressurized gas chamber 120 acts tooverride the default position control being provided for the operatingmandrel assembly 222 by the well test position path 283. Fluidrestriction provided by passage of fluid through the downward flow pathin the fluid metering assembly 142 will provide a sufficiently metereddownstroke so that an operator will have time to repressurize theannulus. It is expected that the time required for the ball sleeveassembly 234 to move fully downward so that the balls 276 essentiallyreturn to ball position a is approximately 10 minutes; the time requiredfor the balls 276 to move only to position c is approximately 4 minutes.It should be apparent to one skilled in the art that the ratchet slot228 and well test position path 283 might be altered such that the balls276 are directed out of the well test position path 283 by an annuluspressure reduction which occurs during an increase of stored fluidpressure in the pressurized gas chamber 120.

A bypass mechanism is included in testing tool with phase change fluidspring 50 which shortens the length of time needed for selected portionsof the metered downstroke to be completed. The bypass mechanism employsthe upper and lower bypass grooves 208 and 232 to selectively permitfluid to bypass portions of the fluid metering assembly at specificpoints during the downstroke to shorten the downstroke. As the annularpiston 210 and affixed bypass mandrel 206 are moved downwardsufficiently, portions of the lengths of upper bypass grooves 208 aredisposed below the upper end 150 and adjacent the clearance 199 andlateral hole 198 of fluid metering assembly 142. In other modes and/orcycles of the testing tool with phase change fluid spring 50, fluidcommunication occurs between the fluid chamber 129 and the upper annulargap 182. The bypass assembly thereby permits fluid from the fluidchamber 129 to bypass the fluid restrictor 196 and move into the secondpassage 188 of the upper fluid flow housing 144 where it may be readilytransmitted downward into the ratchet chamber 170. The downward flow offluid is thereby increased speeding up the downward stroke. By choice ofwidth and length of the upper bypass grooves 208 as well as theplacement upon the bypass mandrel 206, the amount and timing of fluidbypassing may be controlled.

The lower bypass grooves 232, which are located on the upper exterior224 of the ratchet slot mandrel 222, are placed such that, when themandrel 222 is in an upper position, such as in the well test position,the grooves 232 are generally adjacent the annular chamber 179 and nofluid flow occurs therealong. See FIG. 2D. As the mandrel 222 movesdownward with respect to the housing 414, the lower portion of grooves232 are moved adjacent the ratchet chamber 170 and fluid communicationis permitted between the annular chamber 179 and ratchet chamber 170.

When the wellbore annulus is repressured to move the testing tool withphase change fluid spring 50 out of its well test position, the ballsleeve assembly 234 moves upward and balls 276 are moved along slot 228from proximate ball position c to a point above ball position d₁. Theballs 276 have now been directed out of the well test position cycleshown at 283 on FIG. 3 and into a contiguous second ratchet path made upof the remainder of slot 281 to permit the operating mandrel assembly412 to move to alternate mandrel positions wherein the positions of thevalve ball 374 and circulating assembly 416 may be changed. Upwardtravel of the ball sleeve assembly 234 is ultimately limited as shuttlepiston 236 encounters the lower end 152 of the fluid metering assembly142. Downward force is exerted upon the dart 246 permitting upward fluidflow past the check valve 244 and a subsequent reduction in the upwardpressure differential upon the ball sleeve assembly 234. As the pressuredifferential is reduced, balls 276 are shouldered at ball position d₁.

Once the balls 276 have been located at ball position d₁, furtherreduction of the annulus pressure shifts the testing tool with phasechange fluid spring 50 into its blank position with the valve ball 374being moved to a closed position. The operating mandrel assembly 412 ispositioned lower with respect to the ball sleeve assembly and housing414 due to engagement of the balls 276 with the ratchet slot mandrel 222at ball position d₁. The downward pressure differential upon ball sleeveassembly 234 urges it downward along with the operating mandrel assembly412, collet sleeve 318 and ball operating arms 338 to close valve ball374 such that its bore 382 is not aligned with the ball housing bore384. As with other modes and/or cycles, this downward movement is notsufficient to align the circulation ports 296 with the valve apertures304 and permit fluid communication therethrough. As a result, thecirculating assembly 416 remains closed.

During a subsequent well annulus pressure increase and decrease cycle,balls 276 are moved along slot 228 to ball position e₁. This will havethe effect of moving the operating mandrel assembly 412 further downwardwith respect to the exterior housing 414. However, the fluid circulatingassembly 416 remains closed. To prevent damage to the valve ball 374 andits surrounding parts as a result of excessive downward movement of theoperating mandrel assembly 412, protuberances 322 may become disengagedfrom recess 314.

As well annulus pressure is increased and decreased once more, the balls276 are moved from ball position e₁ to position f₁ causing the testingtool with phase change fluid spring 50 to be moved into its circulatingposition. In accordance with other modes and/or cycles, the valve ball374 remains closed and the fluid circulating assembly 416 is opened bythe alignment of the circulation ports 296 and valve apertures 304 topermit fluid communication between the central flow conducting passage56 and the wellbore annulus 46. The testing tool with phase change fluidspring 50 will remain in the circulating position during subsequentannulus pressure change cycles where the balls 276 are movedsequentially to positions f₂, f₃, f₄, f₅, f₆ and f₇.

By way of further explanation of the mode changing and operatingsequence of testing tool with phase change fluid spring 50, the readershould note that the tool only changes mode when balls 276 shoulder atspecific positions on slot 228 during cycling of the tool since ratchetoperation dictates the position of the operating mandrel assembly 412within the housing 414. For example, testing tool with phase changefluid spring 50 changes mode at positions d₁, f₁, f₇, and d₂.

It is also noted that movement between some ball positions is effectedby annulus pressure decrease followed by an increase rather than theincrease/decrease cycle described above. With respect to FIG. 3,specifically, movement from f₆ to f₇, from f₇ to e₂ and from e₂ to d₂ isaccomplished this way.

In addition to the embodiment described above, the phase change fluidspring of the present invention may be used with other downhole testingapparatuses that use a fluid spring to store energy and release storedenergy to operate the testing apparatuses.

The phase change fluid contained in the pressurized gas chamber 120 is afluid that is compressible to a liquid phase preferably at the surface,but that changes to a gas phase or supercritical phase at significantlyhigher pressures when located downhole in the wellbore 4 due to thetemperatures in the wellbore 4. As described above, when phase changefluid spring is disposed downhole in the wellbore 4 it is operable tostore energy then release the stored energy to operate the testing toolin the wellbore 4. As discussed below, a predetermined volume of phasechange fluid is pressurized in the pressurized gas chamber 120 prior tobeing located downhole in the wellbore 4.

Referring next to FIGS. 4A-4B, an embodiment of a phase change fluidspring 450 is shown connected to an apparatus for charging the phasechange fluid spring 450 with such a phase change fluid. The term phasechange fluid as used herein means one or more fluids, elements,substances or mixtures of such fluids, elements or substances that hasthe physical properties of being in a liquid phase at surfacetemperatures and a first pressure such as 1,500 psi or less and thatchanges to a gas phase or supercritical phase at downhole temperatureshaving a corresponding higher pressure, preferably greater than 5,000psi. More preferably the phase change fluid has a pressure in thepressurized gas chamber 120 of from about 8,000 psi to about 25,000 psi,and most preferably from about 10,000 psi to about 20,000 psi, whenlocated downhole in the wellbore 4.

In the illustrated embodiment, phase change fluid spring 450 is chargedwith the phase change fluid on the surface prior to attaching phasechange fluid spring 450 to the testing tool. Phase change fluid spring450 is charged via lateral bore 108 that preferably includes a fittingor connector to connect to a fluid line 456 for receiving a supply ofphase change fluid from a phase change fluid source 454. The pressure ofthe fluid entering phase change fluid spring 450 is monitored andcontrolled by a regulator 452 located between the phase change fluidsource 454 and phase change fluid spring 450.

Preferably, pressurized gas chamber 120 is charged or pressurized withphase change fluid using floating piston 124 to maintain a constantpressure and to assure that the phase change fluid does not changephases during the charging process. This is achieve by pressurizing asecond fluid 458 on the lower side of floating piston 124 when floatingpiston 124 is located at the top of gas chamber 120. The second fluid458 may be an oil or other fluid from fluid reservoir 462 that initiallyfills upper oil chamber 122, longitudinal oil channels 130 and fluidsupply line 460. Fluid supply line 460 is coupled to phase change fluidspring 450 via oil fill port 132. In this manner, the second fluid 458is used to maintain a desired pressure within the pressurized gaschamber 120 while it is being charged or pressurized with the phasechange fluid. Specifically, as phase change fluid enters pressurized gaschamber 120, floating piston 124 moves down acting against the pressureof the second fluid 458. As the pressurized gas chamber 120 is filled,the second fluid 458 may be bled off and captured back in the secondfluid reservoir 462.

In one embodiment, the phase change fluid is in a liquid phase as itenters pressurized gas chamber 120 at the surface. The desired volume ofphase change fluid placed in the pressurized gas chamber 120 may bedetermined using some commonly known gas equations. For example, onesuch equation is the Ideal Gas Equation:

PV=nRt   (I)

where P equals the pressure in atmospheres; V equals the volume of thepressurized gas chamber 120; n equals the number of moles of the phasechange fluid in the pressurized gas chamber 120; T is the temperature inK of the phase change fluid; and R is a gas constant. In one aspect, thegas constant R=0.0821 liter·atmosphere·mole⁻¹·K⁻¹.

Another well known gas equation that may be applied to the determine thevolume of phase change fluid to pressurize in the pressurized gaschamber 120 is the van der Waals Equation of State:

[P+a(n/v)²](V−nb)=nRT   (II)

where P is the pressure in a common unit, such as atmospheres; a is avan der Waals constant in a common unit, such as J·M³/mole²; n is thenumber of moles of phase change fluid; V is the volume in a common unit,such as m³; b is another van der Waals constant in a common unit, suchas m³/mole; R is the gas constant; and T is the temperature in K. In oneaspect, the gas constant R=0.0821 liter·atmosphere·mole⁻¹·K⁻¹. Some vander Waals constants for some substances are noted in Table 1:

TABLE 1 Critical Critical a b Pressure Temperature Substance (J ·m³/mole²) (m³/mole) (MPa) (K) Air 0.1358 3.64 × 10⁻⁵ 3.77 133 Carbon0.3643 4.27 × 10⁻⁵ 7.39 304.2 Dioxide (CO₂) Nitrogen 0.1361 3.85 × 10⁻⁵3.39 126.2 (N₂) Hydrogen 0.0247 2.65 × 10⁻⁵ 1.30 33.2 (H₂) Propane0.5507 3.04 × 10⁻⁵ 22.09 647.3 (C₃H₈) Ethylene 0.4233 3.73 × 10⁻⁵ 11.284.6 (C₂H₄) Propylene 0.00341 2.34 × 10⁻⁵ 0.23 5.2 (C₃H₆) Methanol 1.0789.98 × 10⁻⁵ 4.12 385 (CH₃OH)

The van der Waals Equation of State is a second order approximation ofthe equation of state of a gas that may be used to determine the desiredvolume of phase change fluid for use in pressurized gas chamber 120.Generally, the van der Waals equation works well for temperatures thatare slightly above the critical temperature of a substance. In addition,to these equations, other real gas equations may be used that arecommonly known to those skilled in the art.

Another means of determining the volume of phase change fluid topressurize in the pressurized gas chamber 120 is a pressure-temperaturephase diagram for a particular fluid, element, substance or mixturethereof. For example, referring to FIG. 5, a pressure-temperature phasediagram 470 for a phase change fluid is shown. The pressure-temperaturephase diagram 470 shows a boiling line that is the line that extendsfrom the critical point 480 to the triple point 472, which separates thegas phase or region 482 from the liquid phase or region 476. At thecritical point 480, the densities of the equilibrium liquid phase 476and saturated gas phase 482 become equal resulting in a supercriticalphase 478. As an example, the critical point 480 for the phase changefluid of carbon dioxide occurs at approximately 304.1 K and 73.8 bars.

In the supercritical phase 478, the phase change fluid is above itscritical temperature and critical pressure. The critical point 480represent the highest temperature and pressure at which the phase changefluid, or any supercritical fluid for that matter, can exist as a gasand liquid in equilibrium. Thus, above the critical temperature a gas,such as carbon dioxide, cannot be liquefied by pressure. Nevertheless,at extremely high pressures the fluid can solidify, as shown in FIG. 5.It is noted that the pressures within the pressurized gas chamber 120downhole in the wellbore 4 are to be less than that required to solidifythe phase change fluid. Generally, the inherent characteristics andphase changes near the critical point 480 show large gradients withpressure near the critical point 480. At higher temperatures, the phasechange fluid behaves like a gas at high pressure as can be seen in FIG.5.

As can be seen from the pressure-temperature phase diagram 470, adesired amount of pressure of phase change fluid can be produced for agiven temperature downhole in the wellbore 4. It can be seen that in thesupercritical phase 478 the higher the temperature of the phase changefluid the significantly higher the pressure it produces in thepressurized gas chamber 120, thus enabling the fluid spring operation ofthe present invention.

In one embodiment, the phase change fluid is carbon dioxide. In anotherembodiment, the phase change fluid may be another fluid, element,substance or mixture thereof include, but not limited to, water,ammonia, diethyl ether, methane, ethane, propane, ethylene, propylene,methanol, ethanol, Freon and acetone. The following properties of thesesubstances are noted in Table 2:

TABLE 2 Molecular Critical Critical Weight Temperature Pressure MpaDensity Substance (g/mol) (K) (Atm) (g/cm³) Carbon 44.01 304.1 7.38(72.8) 0.469 Dioxide (CO₂) Water (H₂O) 18.02 647.3 22.12 (218.3) 0.348Methane 16.04 190.4 4.60 (45.4) 0.162 (CH₄) Ethane (C₂H₆) 30.07 305.34.87 (47.1) 0.203 Propane 44.09 369.8 4.25 (41.9) 0.217 (C₃H₈) Ethylene28.05 282.4 5.04 (49.7) 0.215 (C₂H₄) Propylene 42.08 364.9 4.60 (45.4)0.232 (C₃H₆) Methanol 32.04 512.6 8.09 (79.8) 0.272 (CH₃OH) Ethanol46.07 513.9 6.14 (60.6) 0.276 (C₂H₅OH) Acetone 58.08 508.1 4.70 (46.4)0.278 (C₃H₆O)

For making the determination of the volume of phase change fluid to havein the pressurized gas chamber 120, it is important to know severalfactors relating to the downhole conditions in the wellbore 4. Forexample, it is important to acquire what the approximate downholetemperature is where the phase change fluid spring 450 will operate.This temperature can be acquired by any means as is commonly known tothose skilled in the art, such as by downhole temperature gauges and thelike. In addition, a knowledge of the density and depth of the fluidwithin the annulus for determining the hydrostatic pressure of thedownhole wellbore 4. The weight and depth of the mud used in the annulusmay be used to make this determination, for example. Further, the amountof pressure to be cycled on the annulus fluid by the pump 24 and controlconduit 26 is important to making this determination as well.

The following examples are provided to further illustrate the preferredembodiments of the present invention.

EXAMPLE 1

It is determined that a testing tool with phase change fluid spring 50will be used in a particular downhole environment using a phase changefluid of carbon dioxide. The hydrostatic pressure in the annulus may bedetermined by the weight and/or density of the mud and the depth of themud at which the testing toot with phase change fluid spring 50 will beused. For example, if it is determined that the hydrostatic pressure inthe unpressurized annulus is approximately 10,000 psi, then the phasechange fluid in the pressurized gas chamber 120 should sufficientlyexceed the hydrostatic such as a pressure of at least 10,500 psipressure. As described above, energy is stored in the phase change fluidspring 450 by compressing the phase change fluid by pressurizing theannulus using the pump 24 via the control conduit 26. In this example,the amount of phase change fluid to be charged into chamber 120 at thesurface can be determined based upon the required downhole volume usingthe ideal gas law.

In this example, the desired downhole gas volume (V) is 8 liters. Basedupon the desired downhole volume, the downhole temperature and thedownhole pressures, the liquid volume of the carbon dioxide to becharged into the chamber 120 at surface temperature must be determined.In this example, a downhole temperature of approximately 500° C. or773.15 K has been determined using temperature gauges or sensors ascommonly known in the art. The hydrostatic pressure is approximately10,000 psi or 680.5 atmospheres. The gas constant R=0.0821liter·atmosphere·mole⁻¹·K⁻¹. Using these values, it can be determinedthat approximately 85.8 moles of carbon dioxide must be charged as aliquid into chamber 120.

As stated above, the critical point for carbon dioxide occurs at 304.1 K(31.1° C. or 88° F.) and 73.8 bars (1,070 psi). If the charging ofcarbon dioxide into the chamber 120 at the surface is to take place at atemperature of about 88° F., then the pressure on the liquid carbondioxide should be maintained above 1,070 psi, for example 1,500 psi.

Liquid carbon dioxide has a density of approximately 1.03 gms/ml, thus85.8 moles of liquid carbon dioxide, which has a molecular mass of44.0095, will weight approximately 3,432.8 gms. Using the density ofliquid carbon dioxide, this weight of carbon dioxide will occupyapproximately 3,332 mls or 3.33 liters. Thus, second fluid 462 should bemaintained at a suitable pressure to control the rate and volume ofliquid carbon dioxide entering chamber 120. When the desired volume ofliquid carbon dioxide has been charged into chamber 120, lateral bore108 may be closed and phase change fluid spring 450 may be disconnectedfrom phase change fluid source 454 and fluid reservoir 462. In thismanner, the required amount of phase change fluid can be charged intochamber 120 at a pressure significantly lower than the downhole pressureat which it will provide the energy storage capability. In addition,charging the chamber at the lower surface pressure provides for a highdegree of safety during the charging and handling of the phase changefluid spring of the present invention.

EXAMPLE 2

The desired downhole volume (V) of the phase change fluid, in this casecarbon dioxide is 16 liters. The hydrostatic pressure at the desireddepth is approximately 20,000 psi. The downhole temperature at thedesired depth is approximately 250° C. or 523.15 K. The ideal gas law,PV=nRT, may be used to determines the required liquid volume of carbondioxide at the surface. Using the gas constant of R=0.0821liter·atmosphere·mole⁻¹·K⁻¹, it can be determined that approximately253.49 moles of carbon dioxide are required. Charging the chamber 120 atthe surface at a temperature of about 88° F. will require a pressure ofat least 1,070 psi and preferably 1,500 psi to maintain the carbondioxide in a liquid state. Liquid carbon dioxide has a density ofapproximately 1.03 gms/ml, thus 253.49 moles of liquid carbon dioxide,which has a molecular mass of 44.0095, will weight approximately 11,155gms. Using the density of liquid carbon dioxide, this weight of carbondioxide will occupy approximately 11,155 mls or 11.16 liters.

EXAMPLE 3

In another example, the amount of phase change fluid that is chargedinto the phase change fluid spring 450 may determined by weight. Forexample, the change in weight of the phase change fluid source 454 orthe phase change fluid spring 450 may be monitored to determine if therequired amount of phase change fluid has been charged into the chamber120.

EXAMPLE 4

In yet another example, a phase change fluid spring 450 may bepressurized with one or more containers having a known volume of phasechange fluid contained therein. For instance, if it is determined thatapproximately 5 liters of phase change fluid in a liquid state aredesired, then this volume may be charged in the phase change fluidspring 450 using 10 containers or vessels that each containapproximately 500 mls of phase change fluid.

The present invention is described with respect to preferredembodiments, but is not limited to those described. For example, anysubstance may be used that is in a first phase, such as a liquid phase,at the surface at first a pressure and at a second phase, such as a gasphase or supercritical phase, downhole in the wellbore 4. Alternatively,the testing tool with phase change fluid spring 50 might be programmedto effect modes of operation other than those disclosed with respect tothe preferred embodiments described herein. It will be readily apparentto one of ordinary skill in the art that numerous such modifications maybe made to the invention without departing from the spirit and scope ofit as claimed.

1. A tool for use in a testing string disposed in a wellbore and forming an annulus therewith, the tool comprising: a housing defining a central flow conducting passage; a circulating valve disposed within the housing operable to control fluid communication between the central flow conducting passage and the exterior of the housing; a passageway valve disposed within the central flow conducting passage operable to control fluid flow through the central flow conducting passage; and a phase change fluid spring operably associated with the circulating valve and the passageway valve, the phase change fluid spring operates in response to changes in pressure in the annulus, wherein the phase change fluid spring contains a fluid that is in a first phase at a first pressure at the surface and a second phase at a second pressure in the wellbore, the second pressure being greater than the first pressure.
 2. The tool as recited in claim 1 wherein the phase change fluid spring stores and releases energy responsive to changes of annulus pressure.
 3. The tool as recited in claim 1 wherein the first phase is a liquid phase.
 4. The tool as recited in claim 1 wherein the second phase is one of a gas phase and a supercritical fluid phase.
 5. The tool as recited in claim 1 wherein the first pressure is less than about 1,500 psi.
 6. The tool as recited in claim 1 wherein the fluid is at least one of carbon dioxide, water, ammonia, diethyl ether, methane, ethane, propane, ethylene, propylene, methanol, ethanol, Freon, acetone and mixtures of thereof.
 7. A tool for use in a testing string disposed in a wellbore and forming an annulus therewith, the tool comprising: a housing defining a central flow conducting passage; an operating element disposed within the central flow conducting passage operable between two positions, a first position wherein the flow conducting passage through the tool is blocked, and a second position wherein the flow conducting passage is not blocked; a fluid circulating assembly disposed within the housing operable between two positions, a first position wherein fluid communication is allowed between the annulus and the central flow conducting passage and a second position wherein fluid communication between the annulus and the central flow conducting passage is blocked; an operating mandrel assembly slidably disposed within the housing and operably associated with the operating element and the fluid circulating assembly, the operating mandrel assembly operable to move between a plurality of positions such that the operating element and the fluid circulating assembly are actuated to configure the tool into distinct operative modes; and a phase change fluid spring operably associated with the operating mandrel assembly, the phase change fluid spring operates in response to changes in pressure in the annulus, wherein the phase change fluid spring contains a fluid that is in a first phase at a first pressure at the surface and a second phase at a second pressure in the wellbore, the second pressure being greater than the first pressure.
 8. The tool as recited in claim 7 further comprising a pressure conducting channel within the exterior housing, the pressure conducting channel in fluid communication with the phase change fluid spring and the annulus for communicating changes in annulus pressure to the phase change fluid spring.
 9. The tool recited in claim 7 wherein the phase change fluid spring stores and releases energy responsive to changes of annulus pressure.
 10. The tool as recited in claim 7 wherein the first phase is a liquid phase.
 11. The tool as recited in claim 7 wherein the second phase is one of a gas phase and a supercritical fluid phase.
 12. The tool as recited in claim 7 wherein the first pressure is less than about 1,500 psi.
 13. The tool as recited in claim 7 wherein the fluid is at least one of carbon dioxide, water, ammonia, diethyl ether, methane, ethane, propane, ethylene, propylene, methanol, ethanol, Freon, acetone and mixtures of thereof.
 14. A method for actuating a downhole tool disposed within a wellbore, the method comprising: filling a fluid spring with a phase change fluid at the surface, the phase change fluid being in a liquid phase during the filling and being maintained at a first pressure in the fluid spring at the surface; operably associating the fluid spring with the downhole tool; lowering the downhole tool and the fluid spring into the wellbore to a desired depth such that the phase change fluid transitions from the liquid phase to one of a gas phase and a supercritical phase and to a second pressure that is higher than the first pressure; pressurizing the phase change fluid in the fluid spring downhole such that energy to stored in the fluid spring; and releasing the energy stored in the fluid spring to actuate the downhole tool.
 15. The method as recited in claim 14 wherein the step of filling a fluid spring with a phase change fluid at the surface further comprises filling the fluid spring with at least one of carbon dioxide, water, ammonia, diethyl ether, methane, ethane, propane, ethylene, propylene, methanol, ethanol, Freon, acetone and mixtures of thereof.
 16. The method as recited in claim 14 wherein the step of filling a fluid spring with a phase change fluid at the surface further comprises filling the fluid spring such that the first pressure is less than about 1,500 psi.
 17. The method as recited in claim 14 wherein the step of pressurizing the phase change fluid in the fluid spring downhole further comprises increasing the pressure in an annulus surrounding the downhole tool.
 18. The method as recited in claim 14 wherein the step of releasing the energy stored in the fluid spring to actuate the downhole tool further comprises reducing the pressure in an annulus surrounding the downhole tool.
 19. The method as recited in claim 14 further comprising repeating the steps of pressurizing the phase change fluid in the fluid spring downhole and releasing the energy stored in the fluid spring to actuate the downhole tool through a plurality of positions by sequentially increasing and decreasing the pressure in an annulus surrounding the downhole tool.
 20. A fluid spring for actuating a downhole tool in a wellbore, the fluid spring comprising: a housing defining a fluid chamber; and a phase change fluid disposed within the fluid chamber, the phase change fluid in a first phase at a first pressure at the surface and in a second phase at a second pressure in the wellbore, the second pressure being greater than the first pressure.
 21. The fluid spring as recited in claim 20 wherein the first phase is a liquid phase.
 22. The fluid spring as recited in claim 20 wherein the second phase is one of a gas phase and a supercritical fluid phase.
 23. The fluid spring as recited in claim 20 wherein the first pressure is less than about 1,500 psi.
 24. The fluid spring as recited in claim 20 wherein the second pressure is greater than about 10,000 psi.
 25. The fluid spring as recited in claim 20 wherein the fluid is at least one of carbon dioxide, water, ammonia, diethyl ether, methane, ethane, propane, ethylene, propylene, methanol, ethanol, Freon, acetone and mixtures of thereof. 